To clarify the normal value of ventricular wall thickness we directly measured the ventricular wall thickness of 50 normal mongrel dogs of one year old. Because of a wide range in body weight from 6.0 to 16.0 kg of these 50 dogs, we selected 34 dogs weighing 8 to 12 kg to calculate the mean ventricular wall thickness as normal value of one year old. The results obtained were as follows; (1) The mean and standard deviation of the left ventricular wall thickness was 7.1 +/- 1.2 mm at anterior, 6.3 +/- 1.6 mm at posterior, 7.1 +/- 1.3 mm at lateral, 10.3 +/- 1.8 mm at the interventricular septum, and that of the right ventricular wall thickness was 3.4 +/- 1.0 mm. (2) Although any site of the ventricular wall thickness correlated significantly to heart weight, only the anterior left ventricular wall and interventricular septum barely correlated to body weight. (3) There was no significant correlation between arterial blood pressure and the ventricular wall thickness. Further study should be necessary to clarify influences of aging on the ventricular thickness and to determine the criteria of ventricular hypertrophy.